Batteries have limited life spans. When the charge is depleted in a battery, the battery-powered device will cease to function. To circumvent loss of functionality, the battery must be replaced or recharged prior to charge depletion. Accurate determination of a battery's state of depletion is particularly important for battery-powered medical devices that are implanted in human patients. With an accurate determination of battery depletion state, an implanted medical device may be recharged or replaced in order to maintain therapy.
Pulse generators (PGs) are battery-powered medical devices that are implanted in patients and provide electrical pulses (therapy) to stimulate or shock the patient's heart. PGs include cardiac rhythm management (CRM) devices, such as pacemakers, heart failure devices, and defibrillators. Batteries serve as the power source in the PGs, providing power to, for example, generate the pulses, sound a beeper when necessary, and upload recorded data via telemetry. Replacement of the PG when the battery is depleted requires surgery. The consequences of not replacing the PG prior to the point at which battery charge is depleted, however, could deprive the patient of therapy and therefore be life-threatening. Accurate determination of a PG battery's charge supports an effective assessment of the appropriate replacement time, not only allowing the clinician to ensure therapy is available, but also allowing the clinician to maximize time until replacing the PG (i.e., maximize time between surgeries).
Battery voltage is often used as an indicator of remaining battery charge. If voltage is above a threshold, the charge remaining is considered sufficient. Unfortunately, many batteries exhibit aberrant voltage behavior during and following transient periods of increased current draw (e.g., when the beeper is sounding in a PG, when PG telemetry is active, or when a PG charges before delivering a defibrillation shock to the heart). The aberrant voltage behavior is not indicative of the charge in the battery. As such, reliance upon such aberrant voltages can be misleading for purposes of determining battery charge.
To illustrate, FIG. 1 graphically depicts a battery voltage response 102 during a period of high current draw. Voltage is shown with a solid line 102 and current is shown with a dotted line 104. Prior to time tstart, voltage is at an initial, steady-state average level 106, and current is at a relatively low steady-state average level (e.g., 10 μA-100 μA). At tstart, current increases to a high current level 108. In response to the increase in current draw, voltage 102 begins to decrease. During the current draw event, voltage decreases to a low voltage level 110. Current 104 remains at the high level 108 until time tend. At time tend, current 104 returns to a low level, and voltage 102 begins to increase. The time period between tstart and tend is referred to herein as a transient period of increased current draw, or a transient period of decreased voltage.
The period after tend when voltage is returning to a steady-state average value is referred to as the recovery period. Typically, during the recovery period, the voltage increases to a recovery voltage, VR, 112. The recovery voltage VR 112 may or may not be equal to the initial voltage level 106. Illustrated in FIG. 1 are two typical voltage recovery scenarios. A first scenario, depicted with solid line 114, involves voltage overshoot. In the overshoot scenario 114, the voltage 102 increases higher than recovery voltage, VR, 112 and then converges on VR, 112. In the overshoot scenario 114, the voltage may fluctuate around VR 112 before converging. A second voltage recovery scenario, depicted with dotted-dashed line 116, does not involve voltage overshoot. In the second scenario, the voltage 102 continues to increase up to VR 112.
The transient decreased voltage (i.e., the voltage between time tstart and tend) is not considered a valid voltage for purposes of determining the charge remaining, because the voltage is not representative of the steady-state voltage in the battery. In addition, the recovery period for different transient current draw events is variable; it can last minutes, hours, or even days. The length of the recovery period typically depends on the magnitude and duration of the transient increased current draw event, the type of battery, depletion of the battery and other conditions. As a result, an indication of battery charge based on the voltage measured during the transient period and/or during the recovery period may not accurately reflect the true charge held by the battery.
Some products in the past have precluded use of battery voltage measurements made within a fixed 24-hour period of high-voltage charge events. As voltage response to increased current draw can vary considerably with battery type, use of a fixed 24-hour period has the potential to be too short for batteries that exhibit a slow voltage recovery, or needlessly long for batteries that exhibit a long voltage recovery. Using a fixed 24-hour period has the potential to be too short for large magnitude events and needlessly long for small magnitude events. Using a voltage that is still recovering from a large magnitude event or slow recovering batteries may report a voltage that is too low. A low voltage would be interpreted as battery depletion that is greater than actual. Use of a needlessly long period for a small magnitude event or fast recovering batteries could deprive the system of voltage measurements and prevent reporting charge state of the battery. As such, a fixed 24-hour period following only high-voltage charge events does not take into account the either the range of battery types or the range of events that may be encountered.
Thus, a need exists for a method and system for accurately detecting and/or indicating battery charge status despite aberrant voltage response due to a range of transient increased current draw events.